1. Field of the Invention
The present invention relates to a white balance adjusting apparatus, and more particularly to a white balance adjusting apparatus in an image sensing apparatus such as a color video camera, for automatically adjusting white balance to correct the wavelength distribution of light from differing light sources. The adjustment is accomplished in response to color information signal within the image sensing signal obtained from an image sensing device.
2. Description of the Background Art
In taking an object using an image sensing apparatus such as a color video camera, the wavelength distribution of light illuminating the object from a light source differs by the type of the light source. For example, the blue components are intensive in light from a light source of relatively high temperature, whereas the red components are intensive in light from a light source of relatively low temperature. It is therefore necessary to correct the wavelength distribution of each light source in order to properly reproduce the color tone of the object itself illuminated with light of the light source on the screen of a color monitor television receiver. This correction is generally called white balance adjustment, where the gain of each color signal is adjusted so that the ratio of the amplitudes of the three primary color signals of red (hereinafter referred to as R), blue (hereinafter referred to as B), and green (hereinafter referred to as G) is 1:1:1.
In conventional image sensing apparatus, the detection of the three primary color signals R, G, and B is carried out according to light around the image sensing apparatus using a sensor provided for each color. However, white balance could not be adjusted correctly with such image sensing apparatus when the light source around the image sensing apparatus (for example, fluorescent light) differs from the light source illuminating the object (for example, the sun), as in the case where an outdoor scene is taken from inside a room.
Recently, a method called TTL (through-the-lens) is proposed in which white balance adjustment is carried out, without providing separate sensors, according to color difference signals R-Y and B-Y within the image sensing signal obtained from an image sensing device. Such a method is disclosed in Japanese Patent Laying-Open No. 62-35792, for example. This method is based on the consideration that the object taken by an image sensing apparatus has various color area distribution (hereinafter referred to as the color distribution) and if this color distribution is averaged over a sufficient long time, the color components cancel each other to result in each color signal on "0", which is equivalent to a state of taking a completely white picture. By controlling the gains of respective color signals so that the values resulting from integration of color difference signals R-Y and B-Y over one field period, for example, become 0, to correct the offset of the color tone due to wavelength distribution of light of the light source is corrected.
FIG. 1 is a block diagram showing an example of a conventional white balance adjusting apparatus by the TTL method. Referring to FIG. 1, light from an object (not shown) enters an image sensing device 2 formed of a CCD via a lens 1. The incident light is converted by a photoelectric device into an electric signal and provided to a color separating circuit 3. Color separating circuit 3 extracts the three primary color signals of R, G, and B from this electric signal. The extracted G signal is directly provided to a camera processing and matrix circuit 6. The R signal and B signal are provided to camera processing and matrix circuit 6 via a variable gain R amplifying circuit 4 and a B amplifying circuit 5, respectively. Camera processing and matrix circuit 6 creates a luminance signal Y and color difference signals R-Y and B-Y according to the three primary color signals of G, R, and B. The outputs are provided to a video circuit 7 where luminance signal Y and color difference signals R-Y and B-Y are subjected to the well-known process to create a recordable video signal. This recordable video signal is provided to a video recording circuit not shown.
The two color difference signals R-Y and B-Y are applied to integrating circuits 18 and 17, respectively, to be integrated over a sufficient long time, for example over 1 field period of a video signal. The values resulting from the integration are provided to gain control circuits 13 and 14. Gain control circuits 13 and 14 control the variable gains of B amplifying circuit 5 and R amplifying circuit 4 so that the values resulting from integration each becomes 0. This results in the amplitude ratio of 1:1:1 of the three primary color signals G, R, and B to adjust white balance.
The conventional white balance adjusting apparatus of FIG. 1, corrects the irregularity of the wavelength distribution due to light of the light source, based on the consideration that colors cancel each other so that he reproduced picture can approximate a substantially white picture if the various color distributions of the object itself are averaged over a long period. This method is inaccurate when white balance regarding the object itself can not be achieved because the reproduced picture can not approximate a white picture even if the color distributions of the object included in the entire picture are averaged. This arises when the area ratio of the three primary colors within the picture is not equal, that is to say, when the color distribution is not even, such as in the case where green lawn or a blue sky occupies a large area of the picture, or in the case where a human object wearing a red sweater is taken in a close-up manner. If the above mentioned white balance adjustment is applied to such an unbalanced state of white balance, the gain will be controlled so as to cancel the intensive color. In the case of a close-up of a person wearing a red sweater, white balance will be intense in blue, resulting in the color of the object being improperly reproduced on the screen.
This problem is described theoretically hereinafter. FIG. 2 is a graph showing the changes of color difference signals R-Y obtained in the case where a light of the light source illuminates a white color object as the color temperature of the light source changes. The ordinate indicates the red color difference signal R-Y whereas the abscissa indicates the blue color difference signal B-Y. As the color temperature of the light source changes, the color information of the screen, i.e. , the obtained color difference signals vary only within a distribution range in the vicinity of a fixed locus (called the light source color temperature axis) crossing the origin, i.e. the white region.
In ordinary image sensing situations, there are many cases where an object of chromatic colors having color information not in the distribution range, for example, an object having colors such as green, yellow, or magenta, occupies a large area of the picture. The obtained color information, i.e., the color difference signals not within the distribution range do not consider the light source color temperature and are not appropriate as color information for white balance adjustment. It is desirable that these color difference signals are not considered.
The light source color temperature axis crosses the origin (white color), as shown in FIG. 2, where the color difference signal of red is reduced as the color difference signal of blue increases (the fourth quadrant), and the color difference signal of red increases as the color difference signal of blue is reduced (the second quadrant). The first and third quadrants do not include the light source color temperature axis, that is to say, the color information of a region with particularly high chroma does not consider the light source color temperature and is not adequate as the fundamental information of white balance adjustment. This means that when an object having significantly high chroma is included in the picture, the color distribution average of the entire picture does not show an achromatic color due to the effect of high chroma. The unnecessary white balance adjustment causes the white balance to be intense in the complementary color of the high chroma color. Thus, the color of the object cannot be properly reproduced.
The situation where the green portion such as the lawn or a plant occupies a large area on the picture is very common in the case of taking a high chroma object. When the color distribution of the object is intense in green, the color difference signals R-Y and B-Y obtained are both negative values to be included in the third quadrant of FIG. 2. Such color difference signals do not consider the color temperature of the light source and should not be regarded in white balance adjustment. This situation of taking green objects may occur frequently, necessitating a particular solution.